a. Field of the Invention:
The present invention concerns a paste-type lead-acid storage battery, and more particularly it relates to the cell electrode structure of such a battery, and still more specifically it pertains to improved separators for use in such a cell electrode structure.
B. Description of the prior art:
Paste-type lead-acid storage batteries which will be briefly called lead storage battery hereinafter in this specification have heretofore been widely used as the power sources for starting the engines of ordinary type automobiles. However, as the exhaust gases from automobiles powered by internal combustion engines have, of late, become the subject of severe criticism as being a major source of air pollution, lead storage batteries are now freshly attracting the interest of the public as being suitable for the power source of electric cars. For this reason, there is a growing demand for the provision of a lead storage battery exerting an improved performance with respect to its electric capacity and service life. Many attempts have been made in the past to improve the electrode assembly of the cells. For example, the known separators which are placed between the positive and the negative plates of a unit cell for preventing the occurrence of short circuit vary widely in type. There have been used and tested separators made of a micro-porous thin sheet produced by impregnating pulp with a resin and by extruding this impregnated pulp into a sheet form, and more recently those separators which are prepared by the use of a base sheet material made of, for example, porous rubber or sintered polyvinyl chloride synthetic fibers and coating such a sheet with an acid-resistive, oxidation-resistive resin and obtained as a micro-porous film. However, these known separators are still far from being satisfactory in their functions when used in lead storage batteries for electric cars.
That is, the separator which is composed of pulp has insufficient resistance to acids and to oxidation and shows a high electrical resistance. Such a separator can not be expected to have a highly improved electric capacity and a long service life. On the other hand, the separator made of a rubber-like porous material and the separator made of a sintered polyvinyl chloride material both have sufficient resistance to acids and oxidation. However, they have a thickness of about 0.2 to 0.5 mm and a considerably high electrical resistance so that when they are applied to lead storage batteries of electric cars, there is the economical inconvenience that the electrode cell units have to be provided in a large number to obtain a high output.
The separator prepared in the form of film which is made of synthetic fibers as the basic material is desirable since it can be produced as an extremely thin film of about 0.1 to 0.3 mm in thickness and has a low electrical resistance. However, such a separator bears the problem such that the permeation of the electrolytic solution, i.e. sulfuric acid, is hindered at the time the battery is discharged and that accordingly there is the difficulty in the retention of sulfuric acid between the micro-porous fiber structure and the positive plate. For this reason, in case it is intended to use the electrode assembly of the "positive plate--glass mat--film separator--negative plate" structure, the glass mat requires to have a thickness of 0.5 mm or more preferably 1.0 mm or more in order to materialize better permeatior and retention of the electrolytic solution. As a result, although this film-like separator per se can be prepared in a very thin sheet form as stated above, the overall distance between the pair of electrodes can not be so effectively reduced as compared with the prior devices, and the battery capacity itself is not satisfactory either.